Wirelessly Powering a Swarm of Robots

October 7, 2008 by Travis Deyle

There has been a lot of discussion recently by Intel's CTO (Justin Rattner) about some really compelling future technologies: wireless power and programmable matter (made of catoms). Of course, the programmable matter (catoms) he is discussing are basically robots operating as a swarm. Wouldn't it be neat to see the swarms actually powered wirelessly? While Intel has thus far worked on the two technologies disjointly, work presented by myself at ICRA 2008 is addressing the intersection -- wirelessly powering a swarm of robots (publication here).

Wireless power transfer is nothing new; it has been discussed since Tesla's patent in 1900 entitled "Apparatus for Transmission of Electrical Energy" (USPTO #649,621). However, as the technology matures, it will be interesting to see what myriad applications arise.

For example, Intel's system is capable of transmitting up to 60 Watts (the demo of lighting an incandescent bulb) around 75% efficiency. They hope to one day use the system to remotely charge laptops.

Meanwhile for robotics, wireless power has the capability to transform research and applications involving swarms of small (mini/micro) robots. When dealing with robot swarms numbering in the hundreds or even just a dozen, tethering is impractical and changing batteries is cumbersome. By way of an example, consider the battery-powered robot swarm by Caprari from EPFL in Switzerland, where they have ~100 robots operating simultaneously.

Hooking up 100 robots for battery charging does not sound like such a fun prospect -- more importantly, it is a significant research impediment. Instead, it is possible to use inductively-coupled wireless power, in a manner similar to Low-Frequency (125kHz) RFID, to power the swarm or just to perform simultaneous, contact-less battery recharging. This is the goal of the work presented by Deyle (myself) and Reynolds in Surface based wireless power transmission and bidirectional communication for autonomous robot swarms. What's great about this work is that the fundamental design is straight-forward, and can be prototyped using discrete components and basic microcontrollers, making the technology available to researchers and hobbyists alike.

The surface is outfitted with a transmit coil. This coil operates in resonance to increase the circulating current and thus increase the magnetic flux -- providing increased power density on the surface.

The magnetic flux is coupled into a receive coil on the bottom of the robot to provide power.

Now all of the robots can either operate simultaneously on the surface, or they can sit still and recharge on-board energy storage (such as a large capacitor or battery)! Here is what it looks like in operation (along with a wirelessly-powered LED).

For more extensive details, I recommend checking out the publication, here.

Comments

Sure. Only the coils on the transmit side (the surface) are resonant. To build this, you essentially build a transformer with a non-resonant primary (a coil of wire) and a resonant secondary (another coil of wire connected to a capacitor).

In the picture of the upside-down surface, you can see the primary (about 1/4 of the area of the surface). It is loosely coupled, by proximity, to the larger secondary coil that follows the circumference of the power surface. The secondary is connected to four high-voltage capacitors (two parallel strings, each string having two capacitors in series). The capacitors are chosen to achieve the desired resonant frequency -- a function of the coil's inductance and the capacitors' capacitance.

To actually perform charging, place another coil connected to a load (such as a resistor) on the surface. If you have an oscilloscope, you should see an AC voltage across the resistor terminals. This power is being transferred to the resistive load. You can make more elaborate loads that rectify this AC voltage to provide DC. Then you can use the energy to power a load of your choice -- such as a robot!

1. The bidirectional communication refers to communication between the power surface (main large coil) to all the robots, and communication from an individual robot to the power surface. The former is accomplished by modulating the coil voltage / current, while the latter is accomplished by load modulating each robot's receive coil.

2. We used a multimeter to measure the approximate load current / voltage for the robots. For the contour plots in the paper, the load was just a resistor.

3. There are actually two PIC microcontrollers. One of them is in the power surface and is responsible for generating the 125 kHz (measured 112kHz) excitation frequency. The PIC's clock oscillates at ~40MHz, from which it generates the 125kHz drive signal via pulse-width modulation (PWM). The second PIC microcontroller serves as the robot's "brain." It is indeed responsible for the line-following behavior.

I hope this helps clear things up; however, I imagine that the paper is probably the most succinct source of information.

The overall input to the power surface is DC (12 volts, approximately 1 amp if memory serves). The "Power Control Circuit" contains a microcontroller, one of whose digital IO pins switches the power MOSFET on or off. This on / off switching chops the DC into AC in order to drive the primary coil (the power surface). It is important that the coil be driven with AC, as this is what generates the magnetic flux that facilitates wireless communication and power.

The voltage (and current) will depend on the characteristics of your devices. Your current will depend on the resistance of your coils. The voltage across the capacitors (which is actually a function of the quality factor of your circuit) needs to be lower than the rated maximums (note that this can be quite a large voltage if the quality factor is large!). And perhaps most importantly, the frequency of your AC needs to match the resonant frequency of the inductor-capacitor network.

You are correct in that you can drive the entire thing from an AC source. Determining component values and circuit design is left as an exercise to the reader. ;-)

I thought of building a "Charger pad" of my own (hope u know the new product)

I like to charge my mobile phones, laptops, ipods etc so i thought of building a surface just like urs and charge my mobile phone or ipods. So I thought of supplying coils with direct AC voltage by using step down transformer.

What do u think i need to use my supply to the surface to generate magnetic field.

Yes, the resistor is there to represent the FET's internal resistance and the coil resistance. It might make sense to place a physical resistor there if you want to limit the system's power draw or to aid in RX signal properties.

It's pretty tough to accurately measure the circulating current on the resonant secondary -- a current measurement device attached to the secondary would drastically
increase the resonant circuit's resistance, destroy the quality factor (Q) of the circuit, and drastically decrease the circulating current. Instead, you can directly measure the quality factor (Q), from which it should be possible to calculate the circulating current given the system's parameters (eg. resistance, inductance, and/or capacitance). [I leave this as an exercise, since I haven't the time to derive the quantities at this time. Please leave your findings in a follow-up?] To measure the Q, we measured the half-power bandwidth of the circuit under load (see Wikipedia article). In a nutshell... we placed a small coil on the surface at a constant location and only varied the chopping frequency. The resonant frequency produced the largest response in the small coil; we changed the chopping frequency to locate the half-power frequencies and determine the bandwidth. This allowed us to compute / measure a loaded Q of 117 -- see our paper for details (Surface Based Wireless Power Transmission and Bidirectional Communication for Autonomous Robot Swarms).

There are lots of considerations that go into the chopping frequency. The physical size of the coil, the wavelength of operation, government regulations (eg, our system was near the LF standard for RFID at 125kHz), etc. All of these things, and especially the system geometry, will affect your efficiency... and it depends on which efficiency you're referring to. In our system, we intentionally used non-resonant receive coils (on each robot), which have very poor coupling efficiency but allow power transfer from 1-transmit to many-receive without complex interactions. Just adding more robots would increase our system efficiency. If you're interested in the coupling efficiency, I'm afraid you'll probably have to look elsewhere for detailed information -- particularly if you're looking at resonant transmit and receive.

Great Project! I was wondering if there is more detailed construction details available. Schematics, Gerbers, etc? I have an interest in how the communications is done. My power transfer requirements are very low- 10mW or so.